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Abstract:

In order to easily and inexpensively manufacture a shaft member provided
with a flange, which is capable of stably yielding excellent bearing
performance, a shaft member (2) is provided with a shaft portion (2a) and
a flange portion (2b) coupled with a lower end surface (2a2) of the shaft
portion (2a). The shaft portion (2a) is formed into a complete circular
shape in cross section, and the flange portion (2b) is provided with a
through-hole (2c) in which a part of a circumferential region is
retracted to a radially outer side with respect to a lower end of the
shaft portion (2a) so as to exhibit a non-circular shape. The shaft
portion (2a) and the flange portion (2b) are coupled with each other in a
state of butting the lower end surface (2a2) of the shaft portion (2a)
and an upper end surface (2b1) of the flange portion (2b) against each
other. With this, a fluid path (11) is formed by opening both end
surfaces (2b1) and (2b2) of the flange portion (2b) while being
constituted by the following: an upper end opening portion defined by an
upper end opening portion of the through-hole (2c) and a lower end
portion of the shaft portion (2a); and a lower end opening portion
constituted by a lower end opening portion of the through-hole (2c).

Claims:

1. A fluid dynamic bearing device, comprising: a shaft member having a
shaft portion at one end of which a flange portion is provided; a radial
bearing gap formed with an outer peripheral surface of the shaft portion;
and a thrust bearing gap formed with an end surface of the flange
portion, wherein: a through-hole is provided in the flange portion; the
flange portion and the shaft portion are coupled in a state of butting an
end surface of the flange portion and an end surface of the shaft portion
against each other around the through-hole; and a fluid path is formed by
opening both the end surface and another end surface of the flange
portion through the through-hole.

2. A fluid dynamic bearing device according to claim 1, wherein a part of
a circumferential region of the through-hole is positioned on the end
surface of the flange portion on a radially outer side with respect to
the end surface of the shaft portion coupled with the flange portion.

3. A fluid dynamic bearing device according to claim 1, wherein the
through-hole is formed into a non-circular shape.

4. A fluid dynamic bearing device according to claim 1, wherein the
through-hole is formed by press working.

5. A fluid dynamic bearing device according to claim 1, wherein a butting
portion between the end surface of the flange portion and the end surface
of the shaft portion is subjected to laser welding.

6. A fluid dynamic bearing device according to claim 5, wherein a laser
beam is applied from an opening portion on a side opposite to the shaft
portion of the through-hole so as to couple the flange portion and the
shaft portion with each other.

7. A fluid dynamic bearing device according to claim 1, further
comprising: an outer member opened at both axial ends thereof; and a lid
member closing an opening portion at one of the axial ends of the outer
member, wherein: the outer member is provided with: a bearing portion so
that a radial bearing gap is formed between the outer member and an outer
peripheral surface of the shaft member; and a retaining portion for
retaining the bearing portion, which comprises an attachment portion with
respect to a motor base; and the lid member is fixed to an outer
peripheral surface of the outer member.

8. A fluid dynamic bearing device according to claim 7, wherein the
retaining portion of the outer member is formed by injection molding
while the bearing portion is inserted therein.

9. A fluid dynamic bearing device according to claim 7, wherein the outer
member is integrated with the bearing portion and the retaining portion.

Description:

[0002] A fluid dynamic bearing device is a bearing device which rotatably
supports a shaft member with oil films formed in bearing gaps. The fluid
bearing device has characteristics such as high-speed rotation operation,
high rotational accuracy, and quietness. In recent years, by taking
advantage of those characteristics, the fluid bearing device is suitably
used as a bearing device for a motor to be mounted to various electrical
apparatuses such as information apparatuses. Specifically, as a bearing
device for a motor, the fluid bearing device has been suitably used in
the following: a spindle motor for a magnetic disk drive such as an HDD,
an optical disk drive for a CD-ROM, CD-R/RW, DVD-ROM/RAM, or the like; a
polygon scanner motor of a laser beam printer (LBP); or a fan motor for a
PC or the like.

[0003] The fluid dynamic bearing device mounted to the spindle motor among
the above-mentioned motors includes a shaft member and fixation side
members including a bearing sleeve having an inner periphery along which
the shaft member is inserted. Examples of the shaft member used in many
cases include one provided with a flange portion at one end of a shaft
portion thereof. In those cases, a radial bearing gap of a radial bearing
portion is formed between an outer peripheral surface of the shaft
portion and a surface opposed thereto, and a thrust bearing gap of a
thrust bearing portion is formed between an end surface of a flange
portion and a surface opposed thereto (refer to Patent Document 1, for
example).

[0004] As the shaft member provided with a flange, the following are used:
one adopting an integrated type in which both the shaft portion and the
flange portion are formed integrally with each other by machine work such
as cutting; and one adopting a separate type in which the shaft portion
and the flange portion separately prepared are integrated with each other
by an appropriate means. Examples of the shaft member provided with a
flange of the separate type include one in which the one end of the shaft
portion is fixed by press-fitting into a through-hole provided at a
center of the flange portion as described, for example, in JP 2001-317545
A (Patent Document 2).

[0005] Incidentally, during operation of the fluid dynamic bearing device
of this type, pressure balance of a lubricating fluid such as a
lubricating oil, which fills an inner space, may be disturbed. When the
disturbance of the pressure balance occurs between a space defined by one
end surface of the flange portion (first thrust bearing gap, for example)
and a space defined by another end surface of the flange portion (second
thrust bearing gap, for example), there arises a problem of deterioration
in rotational accuracy in thrust directions. Problems of this type can be
solved by providing a fluid path formed by opening both end surfaces of
the flange portion so as to cause a lubricating oil to communicate
between the above-mentioned two spaces through the fluid path.

[0008] In order to provide a fluid dynamic bearing device excellent in
rotational accuracy and reliability, it is important to sufficiently
enhance the accuracy between the shaft portion and the flange portion
(perpendicularity or the like) in advance. While the shaft member
provided with a flange of the integrated type is capable of meeting the
requirement at high level, it is necessary to establish a dedicated
processing installation therefor, which requires tremendous manufacturing
cost. Further, in order to provide the above-mentioned fluid path in the
shaft member provided with a flange of the integrated type, it is
necessary to form a through-hole in the flange portion by drilling
separately effected, for example, after completion of the shaft member.
However, leaving chips produced as a result of the drilling as they are
may lead to a problem of contaminants, and hence it is necessary to
prepare a washing step after the drilling so as to carefully remove the
chips. Therefore, an additional increase in manufacturing cost is
inevitable.

[0009] Meanwhile, the shaft member provided with a flange of the separate
type can be inexpensively manufactured in comparison with that of the
integrated type. By means of press-fitting as described in Patent
Document 2, perpendicularity between the shaft portion and the flange
portion after press-fitting is influenced by working accuracy of the
surface subjected to press-fitting. Thus, at the time of fixing the shaft
portion and the flange portion to each other, it is necessary to finish
not only an outer peripheral surface of the shaft portion, which
constitutes one of surfaces forming the radial bearing gap, but also an
inner peripheral surface of the flange portion in advance with high
accuracy, which leads to an increase in manufacturing cost.

[0010] The present invention has been made in view of the problems
mentioned above, and it is therefore an object of the present invention
to easily and inexpensively manufacture a shaft member provided with a
flange, which is capable of stably yielding excellent bearing
performance.

Means for solving the Problems

[0011] In order to solve the above-mentioned problems, the present
invention provides a fluid dynamic bearing device, including:

[0012] a shaft member having a shaft portion at one end of which a flange
portion is provided;

[0013] a radial bearing gap formed with an outer peripheral surface of the
shaft portion; and

[0014] a thrust bearing gap formed with an end surface of the flange
portion, in which:

[0015] a through-hole is provided in the flange portion;

[0016] the flange portion and the shaft portion are coupled in a state of
butting an end surface of the flange portion and an end surface of the
shaft portion against each other around the through-hole; and

[0017] a fluid path is formed by opening both the end surface and another
end surface of the flange portion through the through-hole. Note that,
the "state of butting against each other" in this case represents a state
of opposing the end surface of the flange portion and the end surface of
the shaft portion to each other, and conceptually includes not only a
state of holding the end surfaces in contact with each other but also a
state in which the end surfaces are partially kept out of contact with
each other.

[0018] According to the above-mentioned structure of the present
invention, merely by providing the through-hole in the flange portion,
specifically, merely by providing the through-hole around which the end
surface of the shaft portion can be butted, and by coupling the flange
portion and the shaft portion with each other in a state of butting the
end surface of the shaft portion against the end surface of the flange
portion, it is possible to form the fluid path. In the above-mentioned
structure of the present invention, when the through-hole provided in the
flange portion is formed to be larger in diameter, the fluid path is
increased in diameter in accordance therewith. Therefore, it is possible
to cause the lubricating fluid to flow into the fluid path more easily,
and possible to reduce fluid resistance generated when the lubricating
fluid flows in the fluid path. Accordingly, the lubricating fluid can be
smoothly communicated between two spaces formed at both the ends of the
flange portion. Even in the case where the disturbance of the pressure
balance occurs in the lubricating fluid between the two spaces, it is
possible to immediately solve the problem of the disturbance of the
pressure balance.

[0019] Further, with the structure of the present invention, in which the
shaft portion and the flange portion are coupled in the state of butting
both the end surfaces thereof against each other, perpendicularity
between the shaft portion and the flange portion can be controlled with
jigs (dies) used at the time of coupling the shaft portion and the flange
portion with each other. Thus, it is possible to reduce accuracy required
for each of the shaft portion and the flange portion, and possible to
inexpensively mass-produce shaft members of high accuracy.

[0020] The above-mentioned structure can be obtained by positioning a part
of a circumferential region of the through-hole on the end surface of the
flange portion on the radially outer side with respect to the end surface
of the shaft portion coupled with the flange portion. Specifically, the
above-mentioned structure can be obtained, for example, by forming the
through-hole into a non-circular shape and forming at least the one end
portion of the shaft portion, which is to be coupled with the flange
portion, into a complete circular shape in cross section so as to be
smaller in diameter than the maximum inner diameter of the through-hole,
and by coupling the one end portion of the shaft portion with the flange
portion after centering on the shaft portion and the flange portion is
completed.

[0021] The through-hole to be provided in the flange portion can be formed
by press working (punch working). With this, the through-hole having a
predetermined shape can be inexpensively formed.

[0022] A coupling means for the shaft portion and the flange portion can
be arbitrarily selected as long as the shaft portion and the flange
portion can be coupled with each other with predetermined strength. Thus,
it is possible to adopt various well-known means such as bonding,
friction welding, and welding. In this context, it is suitable to adopt
welding by which high coupling strength can be secured therebetween even
when a coupling portion (coupling area) is minute, specifically, to adopt
laser welding. That is, it is desirable that the butting portion between
the end surface of the flange portion and the end surface of the shaft
portion be subjected to laser welding. In this case, in order to prevent
to the maximum extent a situation in which dissolved matters produced in
accordance with application of a laser beam adhere to the outer
peripheral surface of the shaft portion and the end surface of the flange
portion, which form bearing gaps, it is desirable that a laser beam be
applied from an opening portion on a side opposite to the shaft portion
of the through-hole so as to couple the flange portion and the shaft
portion with each other.

[0023] The fluid dynamic bearing device according to the present invention
further includes:

[0024] an outer member opened at both axial ends thereof; and

[0025] a lid member closing an opening portion at one of the axial ends of
the outer member, in which:

[0026] the outer member is provided with:

[0027] a bearing portion so that a radial bearing gap is formed between
the outer member and an outer peripheral surface of the shaft member; and

[0028] a retaining portion for retaining the bearing portion, which
comprises an attachment portion with respect to a motor base; and

[0029] the lid member is fixed to an outer peripheral surface of the outer
member. In this structure, the outer member may be constituted by
integrating the bearing portion and the retaining portion separately
prepared with each other by an appropriate means. In terms of cost
reduction, it is desirable that the outer member be formed by injection
molding of the retaining portion while the bearing portion is inserted
therein, or be integrated with the bearing portion and the retaining
portion. Note that, in embodiments described below, a bearing sleeve 8
arranged on an outer periphery of a shaft member 2 and forming a radial
bearing gap between the bearing sleeve 8 and an outer peripheral surface
2a1 of the shaft member 2 corresponds to the "bearing portion", and the
housing 9 retaining the bearing sleeve 8 on an inner periphery thereof
corresponds to the "retaining portion."

[0030] Note that, in the case of using the outer member constituted by
integrating the bearing portion and the retaining portion separately
prepared with each other by an appropriate means as illustrated, for
example, in FIG. 10, it is possible to form a fluid path between the
inner peripheral surface of the retaining portion and the outer
peripheral surface of the bearing portion. Meanwhile, in the case where
the outer member is formed by injection molding of the retaining portion
while the bearing portion is inserted therein as illustrated in FIGS. 2
and 8, or in the case where the outer member is integrated with the
bearing portion and the retaining portion as illustrated in FIG. 9, it is
difficult to form the fluid path of this type. Thus, in the case of
adopting the structure of the bearing device described above, it is
especially effective to apply the above-mentioned structure of the
present invention, in which the fluid path formed by opening both the end
surfaces thereof is provided to the flange portion.

[0031] Incidentally, when the lid member is fixed to the outer peripheral
surface of the retaining portion in the above-mentioned structure, in
comparison with the case of fixing the lid member to the inner peripheral
surface of the retaining portion (housing) as described in Patent
Document 1 mentioned above, it is possible to increase a fixation area
correspondingly to difference in diameter between the inner peripheral
surface and the outer peripheral surface. In the case of fixing the lid
member to the outer peripheral surface of the retaining portion, it is
necessary to provide a disk-like portion closing the opening portion at
the one of the axial ends of the retaining portion (outer member) and a
cylindrical portion fixed to the outer peripheral surface. The fixation
area with respect to the retaining portion is increased merely by
increasing an axial dimension of the cylindrical portion. Thus, it is
unnecessary to increase the thickness of the lid member. In addition,
even when the cylindrical portion is elongated, a dimension of the entire
length of the bearing device is not influenced. With this, it is possible
to increase detachment resistance of the lid member without influence on
the axial dimension of the bearing device or a bearing span between the
radial bearing portions, and hence possible to stably maintain
predetermined bearing performance.

[0032] Further, with the above-mentioned structure, the lid member fixed
to the outer peripheral surface of the retaining portion can be utilized
as an attachment portion to the motor base. While it is effective to form
the retaining portion by injection molding of a resin in terms of cost,
it is difficult in this case to secure fixation strength required in the
case of bonding and fixing the retaining portion to the motor base
generally made of metal. Meanwhile, the fixation strength can be
satisfied when the retaining portion is made of metal, which involves an
increase in cost in comparison with the case of resin. In this context,
with the above-mentioned structure, it is possible to meet the
requirement of cost reduction by forming the retaining portion with a
resin while satisfying the fixation strength of the fluid dynamic bearing
device with respect to the motor base by forming the lid member with a
metal material excellent in bonding property with respect to the lid
member.

[0033] The fluid dynamic bearing device according to the present invention
has the above-mentioned features, and hence can be suitably used while
being incorporated in a motor including a stator coil and a rotor magnet,
for example, in a spindle motor for a disk drive.

Effects of the Invention

[0034] As has been described above, according to the present invention, it
is possible to easily and inexpensively manufacture a shaft member
provided with a flange, which is capable of stably yielding excellent
bearing performance. With this, it is possible to inexpensively provide a
fluid dynamic bearing device excellent in bearing performance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 is a sectional view conceptually illustrating an example of
a spindle motor for an information apparatus, which incorporates a fluid
dynamic bearing device.

[0036] FIG. 2 is a sectional view of a fluid dynamic bearing device
according to a first embodiment of the present invention.

[0044] FIG. 8 is a sectional view of a fluid dynamic bearing device
according to a second embodiment of the present invention.

[0045]FIG. 9 is a sectional view of a fluid dynamic bearing device
according to a third embodiment of the present invention.

[0046] FIG. 10 is a sectional view of a fluid dynamic bearing device
according to a fourth embodiment of the present invention.

[0047]FIG. 11 is a plan view seen from a lower side of the shaft member
according to another embodiment.

[0048]FIG. 12 illustrates a case where a radial bearing portion is
constituted by a multi-arc bearing.

BEST MODES FOR CARRYING OUT THE INVENTION

[0049] In the following, embodiments of the present invention are
described with reference to drawings.

[0050] FIG. 1 conceptually illustrates a structural example of a spindle
motor for an information apparatus, which incorporates a fluid dynamic
bearing device. The spindle motor is used for a disk drive such as an HDD
and includes a fluid dynamic bearing device 1 for rotatably supporting a
shaft member 2, a disk hub 3 fixed to the shaft member 2, a stator coil 4
and a rotor magnet 5 which are opposed to each other through an
intermediation of, for example, a gap in a radial direction, and a motor
base 6. The stator coil 4 is attached to an outer periphery of the motor
base 6 and the rotor magnet 5 is attached to an inner periphery of the
disk hub 3. A housing 9 of the fluid dynamic bearing device 1 is fixed to
an inner periphery of the motor base 6. One or a plurality of disks (two
in illustration) D such as magnetic disks are held by the disk hub 3, and
the disks D are fixed to the disk hub 3 by a clamping mechanism (not
shown). With this configuration, when the stator coil 4 is energized, the
rotor magnet 5 is rotated by an electromagnetic force between the stator
coil 4 and the rotor magnet 5. With this, the disk hub 3 and the disks D
held by the disk hub 3 are rotated integrally with the shaft member 2.

[0051] FIG. 2 illustrates the fluid dynamic bearing device 1 according to
a first embodiment of the present invention. The fluid dynamic bearing
device 1 includes, as components, the shaft member 2, an outer member 7
arranged on an outer periphery of the shaft member 2 and opened at both
axial ends thereof, and a lid member 10 closing the opening at one end of
the outer member 7. The outer member 7 is constituted by a bearing sleeve
8 corresponding to a bearing portion, and the housing 9 corresponding to
a retaining portion for retaining the bearing sleeve 8 on an inner
periphery thereof. Note that, for the sake of convenience, description in
the following is made on the assumption that a side on which the lid
member 10 is provided is a lower side and a side axially opposite thereto
is an upper side.

[0052] The bearing sleeve 8 is cylindrically formed of a porous body made
of a sintered metal, in particular, a porous body made of a sintered
metal including copper as a main component. The bearing sleeve 8 may be
formed of a porous body other than the sintered metal, for example, of a
porous resin or ceramics, or may be formed of a soft metal such as brass.
Both an inner peripheral surface 8a and an outer peripheral surface 8d of
the bearing sleeve 8 are formed into a shape of radially uniform
cylindrical surface. Further, on inner peripheral edges and outer
peripheral edges on both axial ends of the bearing sleeve 8, there are
formed chamfers 8ei, 8eo, 8fi, and 8fo, respectively.

[0053] As illustrated in FIG. 3, on the inner peripheral surface 8a of the
bearing sleeve 8, cylindrical radial bearing surfaces A1 and A2 are
formed separately from each other at two portions in the axial direction
so that a radial bearing gap is formed between the inner peripheral
surface 8a of the bearing sleeve 8 and an outer peripheral surface 2a1 of
a shaft portion 2a opposed thereto. In the radial bearing surfaces A1 and
A2, there are formed radial dynamic pressure generating portions
constituted by a plurality of dynamic pressure grooves 8a1 and 8a2
arranged in a herringbone pattern. In this embodiment, upper dynamic
pressure grooves 8a1 are formed asymmetrically with each other in the
axial direction with respect to an axial center m (axial center of a
region between the upper and lower inclined grooves), and an axial
dimension X1 of an upper region with respect to the axial center m is
larger than an axial dimension X2 of a lower region. Meanwhile, the lower
dynamic pressure grooves 8a2 are formed symmetrically with each other in
the axial direction, and axial dimensions of the upper and lower regions
are equal to the axial dimension X2 described above, respectively. Note
that, the radial dynamic pressure generating portions may be formed on
the outer peripheral surface 2a1 of the shaft portion 2a opposed thereto,
or may be constituted by a plurality of dynamic pressure grooves arranged
in a spiral pattern or the like.

[0054] As illustrated in FIG. 4, on a lower end surface 8c of the bearing
sleeve 8, a thrust bearing surface B is provided so that a first thrust
bearing gap is formed between the lower end surface 8c of the bearing
sleeve 8 and an upper end surface 2b1 of a flange portion 2b opposed
thereto. On the thrust bearing surface B, there is formed a thrust
dynamic pressure generating portion for generating a dynamic pressure
effect in the first thrust bearing gap. The thrust dynamic pressure
generating portion is constituted by dynamic pressure grooves 8c1 bent in
a V-shape and hill portions 8c2 for partitioning the dynamic pressure
grooves 8c1 alternately arranged in a circumferential direction, and
exhibits a herringbone shape as a whole. Note that, the thrust dynamic
pressure generating portion may be formed on the upper end surface 2b1 of
the flange portion 2b opposed thereto.

[0055] The housing 9 is opened at both the axial ends so as to exhibit a
substantially cylindrical shape, and integrally includes a main body
portion 9a having an inner periphery on which the bearing sleeve 8 is
retained, and a seal portion 9b provided on a radially inner side of an
upper end of the main body portion 9a. An inner peripheral surface of the
main body portion 9a is formed into a shape of radially uniform
cylindrical surface, and an outer peripheral surface thereof is formed
into a shape of stepped cylindrical surface radially small on the lower
side. Accordingly, the main body portion 9a exhibits a mode in which a
thick portion 9a1 formed to be relatively thicker and a thin portion 9a2
formed to be relatively thinner are stacked on each other in the axial
direction. Note that, as described later, in this embodiment, an outer
peripheral surface of the thick portion 9a1 of the housing 9 functions as
an attachment portion with respect to the motor base 6.

[0056] An inner peripheral surface 9b1 of the seal portion 9b is formed
into a shape of a tapered surface gradually reduced downward in diameter,
and a wedge-like seal space S gradually reduced downward in radial
dimension is formed between the inner peripheral surface 9b1 of the seal
portion 9b and the outer peripheral surface 2a1 of the shaft portion 2a
opposed thereto.

[0057] The housing 9 structured as described above is formed by die
molding (injection molding) of a resin material while the bearing sleeve
8 is used as an inserted component. The resin material used for molding
of the housing 9 is not particularly limited as long as injection molding
can be effected. As a base resin, any of the following may be used: a
liquid crystal polymer (LCP); a crystalline resin typified by
polyphenylene sulfide (PPS); and an amorphous resin such as
polyphenylsulfone (PPSU) or polyethersulfone (PES). Note that, while
various fillers may be mixed with the resin material in accordance with
required characteristics, as described later, conductivity is secured in
the lid member 10 in this embodiment. Thus, it is unnecessary to mix a
filler (conductive filler) for imparting conductivity to a resin material
for molding. Note that, the conductive filler may be mixed as long as
moldability or the like of the housing 9 is not adversely affected and
there is no problem in terms of cost.

[0058] As a result of insertion molding of the housing 9, an upper end
surface 8b of the bearing sleeve 8 and the outer peripheral chamfer 8eo
are collectively covered with a resin. In addition, as illustrated in
FIG. 2, when at least the outer peripheral chamfer 8fo at the lower end
of the bearing sleeve 8 is covered with a resin, retention with respect
to the housing 9 can be effected on the bearing sleeve 8. The inner
peripheral chamfer 8ei at the upper end of the bearing sleeve 8 is not
covered with a resin such that positioning of the bearing sleeve 8 is
effected in a die by bringing the inner peripheral chamfer 8ei into
contact with the die at the time of injection molding.

[0059] On a radially outer side of the upper end of the housing 9, which
constitutes a boundary between the main body portion 9a and the seal
portion 9b, corner portions are chamfered. Owing to a chamfered portion
9c thus formed, thickness of the housing 9 is substantially uniformed
over the region from the main body portion 9a to the seal portion 9b.
Thus, deformation of the inner peripheral surface 9b1 of the seal portion
9b, which is caused by molding shrinkage after injection molding, is
suppressed, with the result that shape accuracy of the seal space S is
secured.

[0060] Note that, while a resin material is used as a material for molding
of the housing 9 in this embodiment, this should not be construed
restrictively. For example, low melting metal materials such as a
magnesium alloy or an aluminum alloy may be used.

[0061] The lid member 10 is fixed, for example, by gap-filling bonding to
an outer peripheral surface of the thin portion 9a2 constituting the
housing 9 (main body portion 9a). With this, the lower end opening of the
housing 9 is closed. The lid member 10 is formed of a conductive metal
material, and is formed, for example, through press working of a metal
plate into a bottomed cylindrical shape (cup shape) in which plate
portion 10a having a substantially disk-like shape and a cylindrical
portion 10b having a shape of a cylinder extending upward from the
radially outer end of the plate portion 10a are integrated with each
other. The cylindrical portion 10b is overlapped with a part or the
entire of the radial bearing surface A2 of the bearing sleeve 8
(partially in this embodiment) in the axial direction.

[0062] As illustrated in FIG. 5, on an upper end surface 10a1 of the plate
portion 10a, a thrust bearing surface C is provided so that a second
thrust bearing gap is formed between the upper end surface 10a1 of the
plate portion 10a and a lower end surface 2b2 of the flange portion 2b
opposed thereto. On the thrust bearing surface C, there is formed a
thrust dynamic pressure generating portion for generating a dynamic
pressure effect in the second thrust bearing gap. The thrust dynamic
pressure generating portion is constituted by dynamic pressure grooves
10a11 bent in a V-shape and hill portions 10a12 for partitioning the
dynamic pressure grooves 10a11 alternately arranged in a circumferential
direction, and exhibits a herringbone shape as a whole. The thrust
dynamic pressure generating portion may be formed on the lower end
surface 2b2 of the flange portion 2b opposed thereto.

[0063] An upper end surface 10b1 of the cylindrical portion 10b of the lid
member 10 and a lower end surface 9a11 of the thick portion 9a1 of the
housing 9 are opposed to each other in the axial direction. After setting
of a width of the thrust bearing gap described later, an axial gap
δ1 is formed between both the surfaces 10b1 and 9a11. After the
setting of the width of the thrust bearing gap, the axial gap δ1
may be completely sealed, for example, by adhesive. Further, an axial gap
δ2 is formed between the upper end surface 10a1 of the plate
portion 10a and the lower end surface of the thin portion 9a2 of the
housing 9. It is desirable that a gap width of the axial gap δ2 be
as small as possible so as to reduce a retaining amount of oil in the
bearing device.

[0064] The shaft member 2 is constituted by the shaft portion 2a and the
flange portion 2b protruding on the radially outer side from the lower
end of the shaft portion 2a. The shaft portion 2a is formed of a metal
material excellent in rigidity and abrasion resistance, and in this
embodiment, formed of a stainless steel as a solid shaft over the entire
length so as to have a complete circular shape in cross section. The
outer peripheral surface 2a1 of the shaft portion 2a is formed into a
shape of a radially uniform cylindrical surface except an axial region
provided with a cylindrical recessed portion retracted to a radially
inner side. Further, a lower end surface 2a2 of the shaft portion 2a is
formed as a smooth flat surface free from asperity. Meanwhile, the flange
portion 2b is formed of a stainless steel similarly to the shaft portion
2a so as to have an annular shape in which a through-hole 2c is formed.
Both the end surfaces 2b1 and 2b2 of the flange portion 2b are formed as
smooth flat surfaces free from asperity.

[0065] In a state of butting the lower end surface 2a2 of the shaft
portion 2a and the upper end surface 2b1 of the flange portion 2b against
each other, the shaft member 2 is integrated in a coupling manner through
an intermediation of a coupling portion 12 formed on an inner periphery
of the flange portion 2b, more specifically, formed on the inner
periphery of an upper end of the flange portion 2b. Further, the flange
portion 2b is provided with a fluid path 11 formed by opening both the
end surfaces 2b1 and 2b2. The fluid path 11 is formed by opening both the
end surfaces 2b1 and 2b2 through the through-hole 2c of the flange
portion 2b. A manufacturing method for the shaft member 2 is described
later.

[0066] The fluid dynamic bearing device 1 described above is incorporated
into a motor, for example, by bonding and fixing an outer peripheral
surface of the lid member 10 (cylindrical portion 10b) and an outer
peripheral surface of the housing 9 to an inner peripheral surface of the
motor base 6 (refer to FIG. 1) formed of a metal material such as an
aluminum alloy. In this case, when outer diameter dimensions of the
housing 9 and the lid member 10 are equal to each other, those outer
peripheral surfaces can be reliably fixed to the inner peripheral surface
of the motor base 6. Further, in this case, when the inner peripheral
surface of the motor base 6 is formed to be somewhat radially larger than
the outer peripheral surface of the housing 9, the motor base 6 and the
housing 9 can be bonded and fixed with adhesive filling a radial gap
formed therebetween (gap-filling bonding). In addition, both the lid
member 10 and the motor base 6 are made of metal, and hence the fluid
dynamic bearing device 1 can be fixed to the motor base 6 with high
bonding strength. Note that, when sufficient bonding strength can be
secured between the lid member 10 and the motor base 6, it is not
necessary to bond and fix the housing 9 to the motor base 6.

[0067] When the shaft member 2 is rotated in the fluid dynamic bearing
device 1 structured as described above, the radial bearing gaps are
formed between the radial bearing surfaces A1 and A2 of the bearing
sleeve 8 and the outer peripheral surface 2a1 of the shaft portion 2a
opposed thereto. In accordance with the rotation of the shaft member 2,
oil-film pressure in both the radial bearing gaps is increased by dynamic
pressure effects of the dynamic pressure grooves 8a1 and 8a2. As a
result, radial bearing portions R1 and R2 for supporting the shaft member
2 in the radial direction in a non-contact manner are formed separately
from each other at two portions in the axial direction. Simultaneously,
the first and second thrust bearing gaps are respectively formed between
the thrust bearing surface B provided on the lower end surface 8c of the
bearing sleeve 8 and the upper end surface 2b1 of the flange portion 2b
and between the lower end surface 2b2 of the flange portion 2b and the
thrust bearing surface C provided on the upper end surface 10a1 of the
lid member 10. In accordance with the rotation of the shaft member 2,
oil-film pressure in both the thrust bearing gaps is increased by dynamic
pressure effects of the dynamic pressure grooves 8c1 and 10a11. As a
result, there are formed first and second thrust bearing portions T1 and
T2 for supporting the shaft member 2 in both the thrust directions in a
non-contact manner.

[0068] Further, the seal space S is gradually reduced downward (inner side
of the housing 9) in radial dimension so as to exhibit a wedge-like
shape. Therefore, a lubricating oil in the seal space S is drawn-in
toward the inner side of the housing 9 owing to a drawing-in effect
exerted by a capillary force. Further, the seal space S has a buffering
function of absorbing an amount of change in volume due to change in
temperature of the lubricant oil filling the inner space of the housing
9, and an oil level of the lubricating oil is constantly maintained
within the seal space S within an anticipated range of change in
temperature. The above-mentioned structure allows effective prevention of
leakage of the lubricating oil from the inside of the housing 9.

[0069] Incidentally, in this embodiment, in the dynamic pressure grooves
8a1 provided in the radial bearing surface A1 of the bearing sleeve 8,
the axial dimension X1 of the upper region with respect to the axial
center m is larger than the axial dimension X2 of the lower region.
Therefore, when the shaft member 2 is rotated, a drawing-in force of the
lubricating oil, which is exerted by the dynamic pressure grooves 8a1, is
relatively larger in the upper region in comparison with that in the
lower region. Owing to differential pressure of the drawing-in force
(imbalance of pumping capacity), the lubricating oil filling the gap
between the inner peripheral surface 8a of the bearing sleeve 8 and the
outer peripheral surface 2a1 of the shaft portion 2a is pressed downward.
In this case, pressure becomes higher in a space on the closed side in
the bearing, in particular, in a space on the radially inner side of the
second thrust bearing gap, and hence an excessive upward floating force
acts on the shaft member 2. As a result, it becomes difficult to maintain
balance of a thrust supporting force between both the thrust bearing
portions T1 and T2.

[0070] In this regard, in the fluid dynamic bearing device 1 according to
this embodiment, the fluid path 11 formed by opening both the end
surfaces 2b1 and 2b2 is provided in the flange portion 2b, and hence the
lubricating oil can be communicated between both the thrust bearing gaps
through the fluid path 11. Thus, it is possible to solve a problem of
impaired pressure balance between both the thrust bearing gaps at an
early stage so as to maintain the balance of a thrust supporting force
between both the thrust bearing portions T1 and T2, that is, to stabilize
rotational accuracy in the thrust directions.

[0071] Further, as described above, in the fluid dynamic bearing device 1
according to this embodiment, there is a tendency that the pressure in
the space on the radially inner side of the second thrust bearing gap is
increased. In this case, when the dynamic pressure grooves 10a11
constituting the second thrust bearing portion T2 are arranged in a
spiral pattern of a pump-in type, which have been conventionally used in
many cases, the lubricating oil filling the second thrust bearing gap is
pressed into the radially inner side. As a result, an increase in
pressure is promoted in the space on the radially inner side in the
second thrust bearing gap. In this context, it is desirable that the
dynamic pressure grooves 10a11 constituting the second thrust bearing
portion T2 be formed in a herringbone pattern as illustrated in FIG. 5
because this problem can be avoided. Meanwhile, problems of this type do
not arise in the first thrust bearing portion T1, and hence the dynamic
pressure grooves 8c1 may be formed in the spiral pattern of the pump-in
type instead of the herringbone pattern as illustrated in FIG. 4.

[0072] Next, detailed description is made on a manufacturing method for
the shaft member 2 described above.

[0073] First, the flange portion 2b having the through-hole 2c is
prepared. The through-hole 2c is formed so that a part of a
circumferential region thereof is opened on the upper end surface 2b1 of
the flange portion 2b on a radially outer side with respect to the lower
end portion (lower end surface 2a2) of the shaft portion 2a, which is
coupled with the flange portion 2b. Specifically, as illustrated in FIG.
6A, the through-hole 2c in this embodiment is formed into a non-circular
shape in which the following are alternately arranged in the
circumferential direction: three first circular-arc surfaces 2c1 each
having a circular-arc shape and positioned on the radially inner side
with respect to the outer peripheral surface 2a1 of the shaft portion 2a
after centering on the shaft portion 2a and the flange portion 2b is
completed; and three second circular-arc surfaces 2c2 each having a
substantially semi-circular shape and having a radially outer region
which is partially positioned on the radially outer side with respect to
the outer peripheral surface 2a1 of the shaft portion 2a after centering
on the shaft portion 2a and the flange portion 2b is completed as well.
The through-hole 2c having a non-circular shape as described above can be
formed by effecting press working (punch working) on the flange portion
2b formed into a disk-like shape. By means of press working, the
through-hole 2c having a complicated shape of this type can be formed
inexpensively with high accuracy.

[0074] Next, the flange portion 2b structured as described above and the
shaft portion 2a separately prepared are coupled integrally with each
other after centering thereon is completed. Specifically, first, as
schematically illustrated in FIG. 7A, the shaft portion 2a is inserted
along an inner periphery of a lower die 31 and the outer peripheral
surface 2a1 of the shaft portion 2a is held, and then the upper end
surface 2b1 of the flange portion 2b is butted against the lower end
surface 2a2 of the shaft portion 2a. In this embodiment, the flange
portion 2b is placed on the shaft portion 2a (and lower die 31) in a
manner that both the end surfaces 2b1 and 2a2 are brought into contact
with each other. Next, while effecting centering on the shaft portion 2a
and the flange portion 2b, both the end surfaces 2b1 and 2b2 of the
flange portion 2b are held by the lower die 31 and an upper die 32. Note
that, while not shown, in order to easily effect centering on the shaft
portion 2a and the flange portion 2b with high accuracy, it is desirable
to hold the outer peripheral surface of the flange portion 2b.

[0075] In this case, on the premise that the through-hole 2c in the
above-mentioned mode is formed in the flange portion 2b, when centering
is effected in a state of butting the shaft portion 2a and the flange
portion 2b against each other, circumferential regions in the upper end
surface 2b1 of the flange portion 2b, in which the first circular-arc
surfaces 2c1 are provided, are overlapped with the lower end surface 2a2
of the shaft portion 2a in the radial direction. Meanwhile,
circumferential regions in the upper end surface 2b1 of the flange
portion 2b, in which the second circular-arc surfaces 2c2 are provided,
are not overlapped with the lower end surface 2a2 of the shaft portion 2a
(refer to FIGS. 6A and 6B).

[0076] Then, in a manner of passing a through-hole of the upper die 32 and
the through-hole 2c of the flange portion 2b from the lower end surface
2b2 side of the flange portion 2b, a laser beam 33 is applied from a
laser beam applying device (not shown) onto the upper end inner periphery
of the flange portion 2b, more specifically, onto the regions in the
flange portion 2b, in which the first circular-arc surfaces 2c1
overlapped with the lower end surface 2a2 of the shaft portion 2a in the
radial direction are formed. When the laser beam 33 is applied, an edge
portion on the upper end inner periphery of the flange portion 2b and the
lower end of the shaft portion 2a adjacent thereto are fused with each
other. As a result, as illustrated in FIG. 7B, the coupling portion 12 is
formed by fixing the flange portion 2b and the shaft portion 2a through
welding. Similarly, the coupling portion 12 is formed in a
circumferential predetermined region (part or the entire of the region in
which lower end surface 2a2 of shaft portion 2a and upper end surface 2b1
of flange portion 2b are overlapped with each other, that is, part or the
entire of the region in which the first circular-arc surfaces 2c1 are
formed). Simultaneously when the formation of the coupling portion 12 is
completed, an upper end opening portion of the fluid path 11 defined by
the through-hole 2c of the flange portion 2b and the lower end of the
shaft portion 2a, more specifically, by the second circular-arc surfaces
2c2 of the flange portion 2b and the lower end portion of the shaft
portion 2a, and a lower end opening portion of the fluid path 11 is
defined by a lower end opening portion of the through-hole 2c.

[0077] Note that, examples of the usable laser beam 33 used for formation
of the coupling portion 12 include well-known laser beams of various
types, such as a YAG laser beam, a carbon dioxide laser beam, a
semiconductor laser beam, or a fiber laser beam. Of those, the YAG laser
beam and the carbon dioxide laser beam are suitable in consideration of
economy, welding strength, easiness of welding, or the like. Further, the
laser beam 33 may be applied in any of a continuous mode and a pulse
mode.

[0078] As has been described above, in the fluid dynamic bearing device 1
of the present invention, merely by providing the through-hole 2c in the
flange portion 2b, specifically, merely by providing the through-hole 2c
having a non-circular shape so that a part of the circumferential region
thereof is positioned on the upper end surface 2b1 of the flange portion
2b on the radially outer side with respect to the lower end surface 2a2
of the shaft portion 2a coupled with the flange portion 2b, and by
coupling the flange portion 2b and the shaft portion 2a with each other
in a state of butting the lower end surface 2a2 of the shaft portion 2a
against the upper end surface 2b1 of the flange portion 2b, it is
possible to form the fluid path 11 while opening both the end surfaces
2b1 and 2b2 of the flange portion 2b. In this structure, the lower end
opening portion of the opening portion at both the ends of the fluid path
11 is constituted by the lower end opening portion of the through-hole 2c
provided in the flange portion 2b. Thus, when the through-hole 2c
provided in the flange portion 2b is formed to be larger in diameter, the
lower end opening portion of the fluid path 11 is also increased in
diameter. Therefore, in particular, it is possible to cause the
lubricating oil filling the gap (second thrust bearing gap) formed
between the lower end surface 2b2 of the flange portion 2b and the upper
end surface 10a1 of the lid member 10 to flow into the fluid path 11 more
easily, and possible to reduce fluid resistance generated when the
lubricating oil flows in the fluid path 11. Accordingly, even in the
structure of this embodiment, in which pressure is liable to become
higher in the space on the radially inner side of the second thrust
bearing gap, the lubricating oil can be smoothly communicated through the
fluid path 11 between both the thrust bearing gaps. With this, it is
possible to immediately solve the problem of impaired pressure balance
between both the thrust bearing gaps, and possible to stabilize
rotational accuracy in the thrust directions at an earlier stage.

[0079] Further, with the structure of the present invention, in which the
shaft portion 2a and the flange portion 2b are coupled in a state of the
surfaces thereof being butted against each other, perpendicularity
between the shaft portion 2a and the flange portion 2b can be controlled
with the dies 31 and 32 used at the time of coupling. Thus, it is
possible to reduce accuracy required for each of the shaft portion 2a and
the flange portion 2b, and possible to inexpensively mass-produce shaft
members 2 of high accuracy.

[0080] Further, regarding the disk hub 3 (refer to FIG. 1) fixed to the
upper end of the shaft portion 2a at the time of assembly of a motor,
deficiency in coupling strength may lead to a risk that the shaft portion
2a and the flange portion 2b are separated from each other owing to
pressure applied at the time of fixation of the disk hub 3. As in the
case of the present invention, when the shaft portion 2a and the flange
portion 2b are coupled in a state of butting both the end surfaces
thereof against each other, the pressure applied at the time of fixation
of the disk hub 3 is cancelled through engagement of the lower end
surface 2a2 of the shaft portion 2a with the upper end surface 2b1 of the
flange portion 2b in the axial direction. Therefore, it is possible to
effectively prevent the above-mentioned failures.

[0081] Further, the shaft portion 2a and the flange portion 2b are coupled
integrally with each other by applying the laser beam 33 (laser welding).
Thus, even in the structure of this embodiment, in which a contact area
between the shaft portion 2a and the flange portion 2b is relatively
smaller, it is possible to secure high coupling strength therebetween.
Further, at the time of subjecting the shaft portion 2a and the flange
portion 2b to laser welding, the coupling portion 12 is formed on the
inner periphery of the upper end of the flange portion 2b by applying the
laser beam 33 from the lower end surface 2b2 side of the flange portion
2b. Thus, it is possible to effectively prevent a situation in which
dissolved matters such as metal particles produced at the time of welding
adhere to the outer peripheral surface 2a1 of the shaft portion 2a and
the end surfaces 2b1 and 2b2 of the flange portion 2b, which form bearing
gaps, and bearing performance is deteriorated.

[0082] Description is hereinabove made on the case where the shaft portion
2a and the flange portion 2b are coupled integrally with each other in a
state of holding the lower end surface 2a2 of the shaft portion 2a and
the upper end surface 2b1 of the flange portion 2b in contact with each
other. With laser welding, even in the case where the lower end surface
2a2 and the upper end surface 2b1 are partially butted against (opposed
to) each other in a non-contact state, it is possible to couple the shaft
portion 2a and the flange portion 2b with each other with high accuracy
and high strength. As described above, when the shaft portion 2a and the
flange portion 2b are coupled with each other in a state of holding the
lower end surface 2a2 of the shaft portion 2a and the upper end surface
2b1 of the flange portion 2b in contact with each other in the
non-contact state, it is possible to further reduce the accuracy required
for each of the shaft portion 2a and the flange portion 2b, and possible
to further reduce the manufacturing cost of the shaft member 2.

[0083] Further, while both the shaft portion 2a and the flange portion 2b
are made of stainless steel in this embodiment, high fastening strength
can be secured even between different metals by means of laser welding.
Thus, as long as the strength required for the shaft member 2 can be
secured, the shaft portion 2a and the flange portion 2b may be made of
materials different from each other. For example, while the shaft portion
2a may be made of stainless steel, the flange portion 2b may be made of
copper or the like, and cost reduction of the shaft member 2 can be
achieved.

[0084] Note that, description is hereinabove made on the case where the
shaft portion 2a and the flange portion 2b are coupled integrally with
each other by laser welding in which the laser beam 33 is applied. As
long as predetermined coupling strength can be secured between the shaft
portion 2a and the flange portion 2b, a coupling method therebetween may
be arbitrarily selected. For example, the lower end surface 2a2 of the
shaft portion 2a and the upper end surface 2b1 of the flange portion 2b
opposed to each other may be integrated with each other by interposing
adhesive therebetween, or may be coupled integrally with each other by
friction welding or brazing and soldering.

[0085] In addition, in the fluid dynamic bearing device 1 of this
embodiment, the lid member 10 is fixed to the outer peripheral surface of
the housing 9 (outer member 7). Thus, in comparison with the case where
the lid member is fixed to the inner peripheral surface of the housing as
described in Patent Document 1, it is possible to increase a fixation
area between the lid member 10 and the housing 9 correspondingly to
difference in diameter between the inner peripheral surface and the outer
peripheral surface. Further, by reducing an axial dimension of the thick
portion 9a1 of the housing 9, it is possible to increase an axial
dimension of the cylindrical portion 10b of the lid member 10, and hence
possible to easily achieve a further increase in the fixation area, that
is, enhancement of the fixation strength. In addition, it is unnecessary
to increase the thickness of the lid member 10 in accordance therewith.
Accordingly, detachment resistance of the lid member 10 can be increased
without influence on the axial dimension of the fluid dynamic bearing
device 1 and a bearing span between the radial bearing portions R1 and
R2. With this, reliability of the fluid dynamic bearing device 1 is
increased.

[0086] Further, the lid member 10 is made of metal material, and hence
static electricity charged in accordance with the rotation of the disks D
can be reliably discharged to the ground side through a path constituted
by the shaft member 2, the lid member 10, and the motor base 6. Note
that, in the case of bonding and fixing the lid member 10 and the motor
base 6 to each other, there is a risk that the conductive path may be
blocked by adhesive (normally, an insulant). In such a case, it is
desirable to apply a suitable conductive material when necessary so as to
form a conductive film over the radially-outer lower end of the lid
member 10 and the radially-inner lower end of the base 6.

[0087] When the conductive path is constituted by the lid member 10 as
described above, it is unnecessary to take conductivity of the housing 9
into consideration. Thus, more alternatives become available in selecting
molding materials for the housing 9, and hence degree of freedom is
increased in designing the fluid dynamic bearing device 1. In order to
impart conductivity to the housing 9 made of resin, it is necessary to
mix an expensive conductive filler into the resin material. In the
present invention, it is unnecessary to mix the conductive filler of this
type, or possible to reduce a mixing amount thereof. Thus, an increase in
material cost can be suppressed.

[0088] The present invention is not limitedly applied to the
above-mentioned embodiment. In the following, description is made on
other embodiments of the present invention. Note that, in the other
embodiment described in the following, the portions having the same
structures and functions as those in the above-mentioned embodiment are
described while denoted by the same reference symbols, and redundant
description thereof is omitted.

[0089] FIG. 8 illustrates the fluid dynamic bearing device 1 according to
a second embodiment of the present invention. Similarly to the fluid
dynamic bearing device 1 illustrated in FIG. 2, the housing 9 is formed
by injection molding of a resin while the bearing sleeve 8 is inserted
therein, and the lid member 10 made of metal is fixed to the outer
peripheral surface of the housing 9, specifically, to the outer
peripheral surface of the thin portion 9a2. After setting of the thrust
bearing gaps, an axial gap 51 is formed between the upper end surface
10b1 of the cylindrical portion 10b of the lid member 10 and the lower
end surface 9a11 of the thick portion 9a1 of the housing 9.

[0090] In the fluid dynamic bearing device 1 according to the second
embodiment, a coating portion 9d extending to the radially inner side is
formed at the lower end of the thin portion 9a2 of the housing 9, and
covers not only the outer peripheral chamfer 8fo of the bearing sleeve 8
but also the entire of the lower end surface 8c of the bearing sleeve 8.
In an end surface of the coating portion 9d, there are formed a plurality
of dynamic pressure grooves functioning as the thrust dynamic pressure
generating portion of the first thrust bearing portion T1 (herringbone
dynamic pressure grooves illustrated in FIG. 4, for example). Note that,
the inner peripheral chamfer 8fi at the lower end of the bearing sleeve 8
is not covered with the coating portion 9d.

[0091] As described above, formation of the thrust dynamic pressure
generating portion on the coating portion 9d of the housing 9 allows
omission of the thrust dynamic pressure generating portion formed on the
lower end surface 8c of the bearing sleeve 8. Thus, it is possible to
reduce the thickness in the radial direction of the bearing sleeve 8 in
comparison with that in the embodiment illustrated in FIG. 2. The
thinning allows reduction in oil containing amount of the bearing sleeve
8 made of sintered metal. Thus, it is possible to reduce an oil retaining
amount of the entire of the bearing device, and possible to reduce a
thermal expansion amount of oil at the time of rise in temperature.
Accordingly, it is possible to reduce the volume of the seal space S, and
possible to downsize the entire of the bearing device in the axial
direction by reducing the axial dimension of the seal space S.
Alternatively, it is possible to enhance rotational accuracy in the
radial direction by increasing the span between the radial bearing
portions R1 and R2.

[0092] Note that, the thrust dynamic pressure generating portion of the
coating portion 9d can be formed by die molding simultaneously with
injection molding of the housing 9 when grooves corresponding to the
thrust dynamic pressure generating portion are formed beforehand in the
die for molding the housing 9. Thus, it is possible to omit a forming
step of the thrust dynamic pressure generating portion, and possible to
achieve cost reduction. Further, in accordance with the reduction in
axial dimension of the seal space S, difference in thickness is reduced
between the seal portion 9b and the main body portion 9a in the housing
9, and hence deformation of a resin at the time of molding shrinkage less
likely to occur. Thus, in the fluid dynamic bearing device 1 in this
embodiment, the chamfered portion 9c to be formed on the radially outer
portion at the upper end of the housing 9 (refer to FIG. 2) is omitted.

[0093]FIG. 9 illustrates the fluid dynamic bearing device 1 according to
a third embodiment of the present invention. The embodiment illustrated
in the figure is structurally different from the above-mentioned
embodiments in that the outer member 7 is integrated with the following
illustrated in FIGS. 2 and 8: the bearing sleeve 8 serving as a bearing
portion; and the housing 9 serving as a retaining portion for retaining
the bearing sleeve 8. That is, radial bearing gaps (radial bearing
portions R1 and R2) are formed between the outer peripheral surface 2a1
of the shaft member 2 and the inner peripheral surface of the outer
member 7, and a first thrust bearing gap (first thrust bearing portion
T1) is formed between the upper end surface 2b1 of the flange portion 2b
and the lower end surface of the outer member 7. Note that, the outer
member 7 illustrated in this embodiment may be formed by injection
molding of a resin or metal, or may be molded by forging.

[0094] Description is hereinabove made on the respective fluid dynamic
bearing devices in which the outer member 7 is arranged on the outer
periphery of the shaft member 2 having the flange portion 2b provided
with the fluid path 11, the outer member 7 being formed by injection
molding of the housing 9 while the bearing sleeve 8 is inserted therein,
or being integrated with portions corresponding to the bearing sleeve 8
and the housing 9. As a matter of course, the shaft member 2 described
hereinabove can be used while being incorporated in fluid dynamic bearing
devices according to that disclosed in Patent Document 1 mentioned above,
specifically, incorporated in the fluid dynamic bearing device 1
illustrated in FIG. 10. The fluid dynamic bearing device 1 illustrated in
the figure is structurally different from the fluid dynamic bearing
device 1 illustrated in FIG. 2 in mainly including the housing 9, the
bearing sleeve 8, a seal member 13, and the lid member 10 which are
prepared separately from each other and in fixing the other members on
the fixation side (bearing sleeve 8 and the like) to the inner periphery
of the housing 9 by bonding or the like. In the structure according to
this embodiment, in which the outer member 7 is constituted by fixing the
housing 9 and the bearing sleeve 8 which are separately prepared to each
other by bonding or the like in this manner, it is possible to provide a
fluid path 14 between the inner peripheral surface of the housing 9 and
an outer peripheral surface 8d of the bearing sleeve 8, the fluid path 14
being formed by opening both the end surfaces of the bearing sleeve 8. In
this case, it is possible to cause a lubricating oil to flow and
circulate in the bearing through the fluid path 14, and hence possible to
more effectively prevent a problem of impaired pressure balance.

[0095] Description is hereinabove made on the case where the through-hole
2c having a non-circular shape is provided in the flange portion 2b, and
the shaft portion 2a and the flange portion 2b are coupled with each
other in a state of butting the lower end surface 2a2 of the shaft
portion 2a having a complete circular shape in cross section against the
upper end surface 2b1 of the flange portion 2b, with the result that the
fluid path 11 is formed by opening both the end surfaces 2b1 and 2b2 of
the flange portion 2b. The fluid path 11 may be formed otherwise. For
example, as illustrated in FIG. 10, it is also possible to form a fluid
path 11 similar to the above-mentioned one through formation of the
through-hole 2c having a complete circular shape in cross section in the
flange portion 2b and coupling of the lower end surface 2a2 and the upper
end surface 2b1 with each other in the state of butting the lower end
surface 2a2 of the shaft portion 2a at least the lower end of which is
formed into a non-circular shape in cross section against the upper end
surface 2b1 of the flange portion 2b. Note that, FIG. 10 illustrates a
case in which the lower end outer peripheral surface of the shaft portion
2a is constituted by three circular-arc surfaces 2a11.

[0096] In this case, it is also possible to form a wedge-like radial
bearing gap (wedge-like gap Gr) between the outer peripheral surface 2a 1
of the shaft portion 2a and the inner peripheral surface 8a of the
bearing sleeve 8 through forming not only the lower end portion of the
shaft portion 2a, which is coupled integrally with the flange portion 2b,
but the entire of the shaft portion 2a into a non-circular shape in cross
section, for example, as illustrated in FIG. 11, and through arrangement
of the bearing sleeve 8 having the inner peripheral surface 8a on the
outer periphery of the shaft portion 2a. With this, it is possible not
only to form the fluid path 11 in the above-mentioned mode in the flange
portion 2b, but also to constitute, without provision of dynamic pressure
grooves having a complicated shape in any one of the inner peripheral
surface of the bearing sleeve 8 and in the outer peripheral surface 2a1
of the shaft portion 2a which are opposed to each other, the radial
bearing portions R with multi-arc bearings which are categorized as a
type of fluid dynamic bearings.

[0097] Note that, while not shown, it is also possible to form a fluid
path 11 similar to the above-mentioned one through coupling of the lower
end surface 2a2 and the upper end surface 2b1 with each other in the
state of butting the lower end surface 2a2 of the shaft portion 2a at
least the lower end of which is formed into a non-circular shape in cross
section against the upper end surface 2b1 of the flange portion 2b
provided with the through-hole 2c having a non-circular shape.

[0098] Further, description is hereinabove made on the case where the
radial bearing portions R1 and R2 are constituted by the dynamic pressure
bearings in which dynamic pressure effects are generated by the dynamic
pressure grooves arranged in a herringbone pattern or the like, and the
radial bearing portions R are constituted by multi-arc bearings which are
categorized as a type of fluid dynamic bearings. The radial bearing
portions may be constituted by other well-known dynamic pressure bearings
such as so-called step bearings and corrugated bearings. Alternatively,
the radial bearing portions may be constituted by cylindrical bearings in
which two surfaces opposed to each other through an intermediation of the
radial bearing gap (both outer peripheral surface 2a1 of shaft portion 2a
and inner peripheral surface 8a of bearing sleeve 8 in the embodiments
described above) are formed as cylindrical surfaces.

[0099] Further, in the embodiments described above, description is made on
the case where the thrust bearing portions T1 and T2 are constituted by
the dynamic pressure bearings in which dynamic pressure effects are
generated by the dynamic pressure grooves arranged in a herringbone
pattern or the like. Any one or both of the thrust bearing portions T1
and T2 may be constituted by other well-known dynamic pressure bearings
such as so-called step bearings and corrugated bearings.